Ethylene polymerisation by Ni–diphosphine azine complexes

Abstract

The properties of [NiX(PR2CH2C(But)NNC(But)CH2PR2)]+ complexes (where X = Br, and R = cyclohexyl (Cy), isopropyl (Pri), tert-butyl (But), phenyl (Ph); X = Cl or I, and R = cyclohexyl) as catalysts for the polymerisation of ethylene were evaluated with or without the co-catalysts methylaluminoxane (MAO), diethylaluminium chloride, trimethylaluminium or tri(isobutyl)aluminium. Their efficiency depends on the characteristics of the halogen (X) and the R group of the diphosphine azine ligand. Bromide (X) strongly enhances the catalytic properties of the complexes within the R order Cy > Pri > Ph > But. Temperature, co-catalyst ratio (Al/Ni) and complex concentration also influence the catalytic activity. The best results were obtained with [NiBr{PCy2CH2C(But)NNC(But)CH2PCy2}]Br activated by MAO (A = 25.8 kg (mol Ni)−1 bar−1 h−1). The polymers were characterised using NMR and differential scanning calorimetry as branched polyethylenes, the number of branches increasing with the temperature of polymerisation. The molecular weights of the polymers were estimated using NMR. A proposal for the catalyst active precursor is made on the basis of experimental data and molecular orbital calculations. Copyright © 2007 Society of Chemical Industry